Example embodiments of the inventive concept relate to a transistor and a method of fabricating the same, and in particular, to a field effect transistor and a method of fabricating the same.
With the development of information technology (IT), there is an increasing demand for semiconductor devices with higher density, higher speed, higher voltage, and higher current density. Especially, a high electron mobility transistor (HEMT), which is usually a gallium nitride (GaN) device, is widely studied as a high speed, high voltage, and high current density device.
Generally, a reduction of a gate length makes it possible to increase a speed of a semiconductor device. However, the reduction of the gate length may result in deterioration in electric resistance and high frequency characteristics of the device. Further, an increase in integration density of the semiconductor device may lead to reduction of gate-to-drain distance and breakdown voltage. To improve a power density property of the semiconductor device, the gate electrode should be configured to have an increased width, but the increase of gate width may result in a collapse of the gate electrode.
To overcome this problem, the gate electrode is configured to include a narrow leg portion and a wide head portion. For example, the gate electrode is formed to have a ‘T’-, ‘Y’- or mushroom-shaped section. Alternatively, the transistor is configured to include a Γ-shaped gate and a field plate capable of increasing a breakdown voltage of the device. In the Γ-shaped gate, the gate leg portion is formed to have a small width and the gate head portion is formed to be elongated toward a specific direction. Since the head portion of the Γ-shaped gate is wider than the leg portion, it is possible to decrease a total resistance of the gate electrode and to form a depletion region between the head portion and the drain electrode. As a result, a breakdown voltage between the gate and drain electrodes can be increased.
Conventionally, a photoresist layer is formed to have a double- or triple-layered structure, and such a difference in sensitivity between the layers of the photoresist layer is used to form the T-shaped or Γ-shaped gate electrode with a wide head portion and a narrow leg portion. In this conventional gate-forming method, an E-Beam lithography process is used to meet a reduced design rule of the device.
In the case of the Γ-shaped gate, since the wide head portion is provided on the narrow leg portion, the gate may be structurally unstable. Further, the head portion may be asymmetrically provided on the leg portion, and in this case, the semiconductor device may suffer from reliability aggravated by its fabrication process or other factors. For example, the gate may be broken or collapsed by unstable weight distribution or structural instability of the head portion.